Acoustic device



1932 E. H. SMYTHE 1,871,243

ACOUSTIC DEVICE Filed Aug. 15, 1931 IN VE' N TOR E. H. SMYTHE ATTORNEY Patented Adi. 9, 1932 x E 'nwm' Ir. or nvnns'ronjrmmors, ASSIGNOR 'ro BELL TELEPHONE LABORA- PATENT OFFICE TOBIES, INOORPORATED, OF NEW'YORK, ,N. 'Y., A CORPORATION .OF NEW YOItK ACOUSTIC nnvron Application filed August 15, 1931. Serial mi. 557,235.

V n I This invention relates to improvements 1n acoustic, devices and particularly to sound conduits such as usedin horns and sound transmitting apparatus.

The object of the invention is to improve the transmission. of sound waves in acoustic lines. I

Heretofore ed aroundv the ,bends in horns or from one chamber tov another of substantially different cross-sectional area by one channel with the result that the component parts ofthe sound wave front travelled in different times from one section to another because of the difference in the length of the paths in different parts'of the conduit. Conduits having large dimensionsin the plane of abend also cause distortion at high frequencies due to reflection.

In accordance with one feature of the invention the sound waves aretransmitted from one chamber or portion of a conduit to another of larger cross-section without distortin the wave front the sound waves enterin sound waves around a bend and thereby control the wave front so that after it has passed the bend it will be substantially parallel to the plane of the exit opening. In one embodiment the invention pro'vides a plurality of passages arranged spirally around the axis of the bend. v Referring to the drawing:

Fig. 1 is an elevation of a curved horh ema bodying the invention and shows the general type of horn to which the invention is applicable; A Y p Fig 2 is a side'viewof an acoustic conduit of circular cross-section having a 180 curvature and four passageways embodying the invention, the portion of the conduit onone side of the planethrough 2-2 ofFig. 3 being removed to show the shape of these passages;

the sound waves were conduct- I structure shown in Figs. 2 and 3 with two passageways.

Fig. 6 is a side iew of a modification embodying the invention in which a plurality of tubes twisted together form the passages in an acoustic conduit;

Fig. '1 is a front view along the line 7-7 of Fig. 6. T

Fig. 1 shows a curved horn ofthe general type-to which the inventionis applicable cemprisingin order a straight bell portion 20, a 180 bend 21, and a throat portion 22 to which a suitable sound reproducer 23 may be attached. The curved portion 21 is divided into a plurality of spiral-shaped passages which may take the form o fany of the modifications hereinafter described. I

Referring to Figs. 1, 2 and 3, the portion 21 of the horn is divided into four small passages A, B, C and D, of substantially equal length by two helical partitions 24 and 25 extending longitudinally of the'portion 21 and intersecting along the axis thereof. The partitions 24 and 25 turn gradually through 360 about their intersection between the in: let and the outlet of portion 21 thereby dis placing the inlets with respect to the outlets of the passages A, 18,0 and D 360 to equalize the paths of the sound waves.

Figs. .4 and 5 show a modification in which a single partition 26 the centerline of which coincides with the axis of the section 21 and isgradually turned thereabout to reverse the its inlet and outlet of passages E and. F 180 as shown in Fig. 5.

Referring to Figs. 6 and 7 this modification shows the bend made up of a plurality of similar tubes spirally arranged around a core 40 which ma be solid. Although not shown for the sa e of clearness the spaces between the tubes at their ends should be closed by any suitable material, sold'enora cementf or example, to make/the structure air-tight where it joins the straight portions of the horn..

' twist is all that is necessary to equalize their mean lengths as long as the axes of the two passages at their inlet and outlet ends lie in' the plane of the bend. For bends having more than twoassages, the angle of twist should be 360 or any degree of bend; Ob-

viously, the greater the number of passages the more nearly plane will be the wave front at the outlet. Reducing the area of the passages to increase-their number too greatly may introduce friction losses and decrease the volume output unnecessarily; The number and size of the passages will depend-upon the size and re uirements of the horn.

Acoustic. con nits in accordance with the invention have the advantages that they correct-to a large degree the phase displacement of the wave front in curved horns, andthe cross-resonances due to. this displacement. The invention also permits the use of bends same length having inlets in a transverse plane and outlets in a transverse plane, said inlets and said outlets as viewed from their ends being substantially in reverse order.

4. A sound conduit having a curved section comprising a plurality of similar passages, said passages being disposedrin helical fashion around the axis of said curved section.

5. A sound conduit having a curved sectioncoinprising a plurality of substantially 1 equal length ducts disposed symmetrically and in helical fashion around the axis of said curved section.

6. A sound conduit havingva curved sectioncomprising a plurality of substantially equal length ducts, each of said ducts being so disposed about the axis of the curved section that equal portions of its length lie on the short side and on the long side of'the curve.

In witness whereof, I hereunto subscribe I my name, this 21st day of July 19.31. EDWIN H. SMYTHE.-

in horns which are sharper and larger in the.

dimension in the plane of the bend resulting in a more compact structure. Heretofore it hasbeen necessary in order to transmit the high frequencies efiiciently to make such bends substantially flat in the directionof the bend which re uire'd a section to be made comparatively 'Wi efat tlliSPOiIlt. For this frequencies reason the-construction of large horns was.

limited to those of rectangular cross-section.- The present invention permits the use of those of circular cross-section as the high are t faithfully transmitted around'the bends having a plurality of passageways.

What is claimed is: 1. A sound conduit-comprising two sound chambers having different cross-sectional areas, a plurality of uniformly distributed sound outlet openings in one chamber and? an equal number of uniformly distributed inlet; openings in another chamber, and a plurality of channels of equal length connecting said outlets to said inlets, the longitudina axes of saidchannels being oblique to the a longitudinal axes of said chambers.

.2. A sound conduitcomprising a primary portion having diverging walls, a secondary portion having diverging walls, and a third portion connecting said primary and said secondary portions, said third portion compllurality of twisted channels of prising a equal lengt I v 3. A sound conduit having a bend therein comprising a plurality of channels of the llO 

